Mobile and portable electronic systems, such as smartphones, tablet computers, multimedia devices, motor vehicles, airplanes, and various other mobile and portable processor-based systems, employ one or more rechargeable batteries to supply electrical power to their electronic components. While various battery technologies have been utilized over the years, lithium-based batteries (e.g., lithium, lithium-ion, and lithium-polymer) have been chosen more recently for most mobile and portable battery applications. Lithium-based batteries hold their charge longer than other batteries, are recyclable, are lighter-weight than other batteries, are low maintenance (e.g., do not need periodic discharging and have no battery memory), and offer higher voltages and energy densities than other batteries, resulting in less cells needed for desired battery voltages. However, lithium-based batteries are not without their disadvantages. For example, such batteries require thermal protection circuits to maintain safe operation. Such circuits limit the peak voltage or state of charge of each cell during charge and limit maximum current during discharge. In addition, the thermal protection circuits monitor battery or cell temperature to prevent temperature extremes. Allowing a battery to get too hot can have serious consequences, including battery failure and possible safety consequences. Finally, the internal resistances of lithium-based batteries increase with age (e.g., charge-discharge cycles), which causes the voltage at the terminals of a battery to drop under load, reduces maximum current draw, and reduces deliverable energy density. Additionally, the increased resistance associated with aging causes the battery cell to generate more heat during operation for a consistent amount of supplied current.
To monitor the temperature of a lithium-ion battery, a thermal management or protection circuit typically includes a thermistor positioned on a printed circuit board (PCB) adjacent the cell or cells of the battery. The thermistor changes its resistive properties in response to changes in temperature, thereby allowing a voltage applied across the thermistor to change as the temperature changes. The voltage versus temperature characteristics of the thermistor are typically stored in memory and used by a processor to determine the temperature sensed by the thermistor based on the voltage drop detected by across the thermistor.
To illustrate the use of a thermistor to sense the temperature of a battery cell, reference is made to FIG. 1, which depicts a cross-sectional view of a prior art electronic device 101 containing a battery. The electronic device 101 may be a smartphone or other electronic system, and the battery may include one or more cells 103 (one shown). In the configuration depicted in FIG. 1, the battery cell 103 resides under a microprocessor (μP) 109 attached to a circuit board 111. Such a vertical configuration is commonly used in small, lightweight electronic devices, such as smartphones, cellular phones, portable multimedia players, and tablet computers. A thermistor 105 is attached to a circuit board 107 such that the thermistor is positioned proximate the battery cell 103 and forms part of a thermal protection circuit that receives supply power from the cell 103 or an output regulation circuit (not shown).
During operation of the electronic device 101, a hot spot 113 on the battery cell 103 may occur due to self-heating of the cell 103 (e.g., due to the cell's internal resistance) and/or the effect of external heating surrounding the cell 103. For example, heating of the microprocessor 109 or other circuitry near the cell 103 and self-heating of the cell 103 during operation of the electronic device 101 can cause a hot spot 113 to occur on the cell 103. Additionally, depending on the size (area) of the cell 103, the location of the hot spot 113 may be a substantial distance 115 away from the thermistor 105. As a result, the temperature affecting the thermistor 105 (and accordingly sensed by the thermal protection circuit as a result of the change in resistance of the thermistor 105) may be several degrees Celsius lower than the actual hot spot temperature. In the past, batteries were typically encased within an aluminum housing, and the thermistor was thermally coupled to the housing. The high conductivity of the housing allowed the thermistor to more closely track the hottest temperature of the battery. More recently, however, the aluminum housing has been replaced with a lightweight pouch made of aluminum foil encased within a laminate or polymer. Due to the presence of the polymer, the pouch provides poorer thermal conductivity than does the all-aluminum housing. As a result, hot spots more than a few millimeters away from the thermistor may not be accurately detected, and the thermal state of the battery may not be accurately monitored.
In an attempt to account for the poor thermal conductivity of the laminate pouch and the likelihood that cell hot spots may be a substantial distance away from the thermistor, prior art thermal protection systems include hard coded, fixed temperature offsets in their memories. The fixed offsets are typically determined based on lab testing of the thermal performances of the battery cells in the particular configurations of the electronic devices. Once determined, the offset is stored in memory and added by the processor to the sensed temperature (i.e., the temperature determined from the thermistor's voltage versus temperature curve based upon the output thermistor voltage) in order to arrive at the estimated maximum temperature of the cell. While a single fixed temperature offset provides some level of temperature compensation, it does not account for battery aging and, therefore, cannot accurately account for the increase in internal resistance, other changes in the cell's physical and thermal properties, and resultant increase in heating that occur as the battery ages.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated alone or relative to other elements to help improve the understanding of the various exemplary embodiments of the present invention.